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Supernova: How Massive Stars Explode

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  • 0:01 Iron: A Massive Star's…
  • 1:34 Contraction of the Core
  • 3:57 Core Bounce & Explosion
  • 6:34 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will explain how it is that a supernova comes to be by delving into how concepts like iron, core contraction, a core bounce, the strong force, and neutrinos all play a role in this event.

Iron: A Massive Star's Dead End

The next time you peel onions in the kitchen, try not to cry. Instead, smile and think about how an onion represents the inner parts of supergiant stars, for a supergiant star has a layer of onion-like shells where thermonuclear reactions take place. The energy spawned by these shells helps to prop up the star's massive size, but only for so long.

For an element to be a good thermonuclear fuel, energy has to be given off when that fuel's nuclei collide and fuse. The reason this energy is given off is thanks in part to the strong force that super-glues neutrons and protons together. Again, the strong force is a force that binds protons and neutrons inside atomic nuclei.

But protons have positive charges, and they repel one another like two magnets placed with the same poles facing each other would repel one another. Because of this repulsion, an input of energy is necessary to add extra protons to nuclei that are larger than iron (which has 26 protons). Therefore, nuclei of such a size or larger cannot be used as a nuclear fuel to release energy.

Basically, it boils down to this: If you were to peel back a star's nuclear fusion onion, you would go through layer after layer of nuclear fuel that is being consumed by the star. At the very center, an iron rich core would fall out, where no nuclear reactions are taking place.

Contraction of the Core

What I'm about to tell you next is so massive in scope and so complex that not even the most powerful modern computers can fully model or understand it. It is a theory, but a good one nonetheless. As a dying star's core gets hotter and hotter as it contracts more and more, particles called neutrinos escape the star's grasp, draining its energy. A neutrino is a massless and neutral atomic particle traveling near or at the speed of light.

Since the escaping neutrinos are draining the star of energy, the star must compensate for this loss of energy. It does this by contracting, using more of its fuel, or even both. Of course, as you can only imagine, there is a small problem when the core becomes iron. We can't use iron as fuel to give off energy as I have already explained. Therefore, our only remaining option to compensate for the neutrino energy drain is to contract and heat up as a result.

The core in a star with an original mass of eight solar masses will, thus, contract and heat up immensely and extremely quickly, within a tenth of a second. The incredibly hot core gives off highly energetic gamma ray photons that sort of melt the iron nuclei into smaller helium nuclei, meaning this whole thing reverses millions of years of nuclear reactions that built up the iron core in a fraction of a second - that's how incredibly powerful this process is. And just so you're aware or were wondering, a gamma ray is an electromagnetic wave consisting of high photon energy, high frequency, and short wavelength.

Anyway, another tenth of a second later, the core becomes even denser, so much so that it forces protons to combine with the core's electrons to make a huge amount of neutrons - such a process releases a massive amount of neutrinos. To put into perspective as to how dense the core becomes about one fourth of a second after it begins rapid contraction: The earth would have to shrink from a diameter of nearly 8,000 miles to a diameter of only 1,000 feet in the same time span to mimic this density.

The Core Bounce and Explosion

At such a high density, it becomes very hard to compress matter any further. I mean, seriously, if you shrunk the earth by that much, how much further can you actually expect to shrink it? You can't expect the star's core to keep shrinking, either. Therefore, the neutron-rich core becomes very rigid, stops contracting, and the innermost part of the core expands just a bit. This slight expansion, termed a core bounce, causes a very strong wave of pressure upwards and outwards from the inner parts of the core.

If you're having a hard time understanding this bounce, just place a stress ball in your hand. After you compress or contract it and let go, it will spring or bounce back because you over-compressed it. This is what happens with the core. It sort of overdoes the whole compression thing and bounces back just a bit as a result.

At just around this time, matter is diving inward at very high speeds and with a tremendous force. As this material crashes into the now rigid core, it encounters the outward moving pressure wave. The material begins to rush back to the star's surface, in part thanks to the massive amounts of neutrinos that are rushing out of the star's core as well.

The initial pressure wave eventually becomes a faster wave, a shock wave, thanks in part to rising superheated gas delivering an extra jolt of energy, and this wave reaches the star's surface several hours after this entire sequence begins. It then bursts outwards and blasts the star apart. It takes several hours because you should know that these stars are so massive their diameter can be well over a billion kilometers!

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